The Al-Du’aythah volcanic cones, Al-Madinah City: implications for volcanic hazards in northern Harrat Rahat, Kingdom of Saudi Arabia

Abstract

The basaltic Al-Du'aythah volcanic cones lie in the northern part of the extensive lava field of Harrat Rahat, and only 13 km from the centre of Al-Madinah City, in the Kingdom of Saudi Arabia. Historical records indicate they may have erupted in AD 641. The four cones are formed by deposits that record a transition from phreatomagmatic to magmatic explosions followed by minor lava effusion. Three cones display elongated tuff rings at the base, and two produced late-stage lava flows. The cones themselves are symmetrical and constructed mostly by the accumulation of ballistically ejected pyroclasts. Spherical bombs and lapilli (cannonball bombs/lapilli), occasionally with country-rock fragments inside (both cored and loaded bombs/lapilli) are common within the tuff ring deposits. LiDAR data show a total volume of 1,664×10 −6 km 3 for the four cones (418×10 −6 km 3 DRE). Whole-rock chemical analyses indicate alkali-basalt compositions (SiO 2 44.7-45.9 wt%), with little compositional variation and no relationship between chemistry and eruptive styles. Small differences in composition may reflect variations in fractional crystallisation of clinopyroxene and olivine. A magnetotelluric 2D cross-section shows that the cones are located adjacent to a buried sediment-filled alluvial channel along a NNW-SSE fault dipping to the east. The AlDu'aythah eruption was related to the ascent of magma through this structure, with the first phase of the eruption triggered by the interaction of the magma with water from the northern Harrat Rahat aquifer that exists in the AlMadinah basin. This initial water source was rapidly exhausted, while the eruption progressed roughly from north to south and from west to east, the latter motion probably along the fault-controlled feeding dyke. Our work draws attention to the existence of recent explosive phreatomagmatic eruptions in the Al-Madinah basin, which, despite the hyperarid climate of the area, must be considered a potential future eruption hazard.

Review of Explosive Hydrovolcanism

“…Even in lava spatter cone growth dominated by magmatic fragmentation, intermittent explosive magma-water interactions can take place when magma flux drops or the ongoing eruption facilitates new fracture development that drives groundwater toward the active conduit [184,185]. As the general wisdom is that phreatomagmatism is associated with the initial phase of the small-volume edifice growth [186] and its eruptive products also commonly preserved in the basal successions of scoria cones [187]. This scenario is commonly interpreted as the result of the gradual overrun of the magmatic system by environmental aspects such as in eruptions where the magma flux gradually increases over the course of the eruption or where the conduit gradually becomes isolated from the surrounding groundwater, with chilled magma plating the conduit walls [136,188].…”

Volcanic history of the northernmost part of the Harrat Rahat volcanic field, Saudi Arabia

“…Where possible, scoria cone names were used, but for units lacking a scoria cone, names were assigned based on distinguishing geomorphic features or the city district where they were predominantly mapped. Previous assigned names were kept where possible, such as bdu (basalt of al Du'aythah in Murcia et al, 2015). Some previously named scoria cones (i.e., al Anahi) of similar composition are clustered, in which case, we used the previously assigned name and added numeric identifiers with one being the geochronologically oldest (i.e., ban1, ban2, and ban3 for the three scoria cones near al Anahi).…”

“…In addition, many of these volcanic fields remain poorly studied, but growing populations and infrastructure across the region increase the risks from potential future activity (e.g., Lindsay and Moufti, 2014). As a result, the timing, composition, and magmatic processes controlling and influencing activity within these late Cenozoic volcanic fields are being investigated (e.g., Shaw et al, 2003;Moufti et al, 2012Moufti et al, , 2013Duncan and Al-Amri, 2013;Murcia et al, 2015Murcia et al, , 2017Abdelwahed et al, 2016;Duncan et al, 2016;Konrad et al, 2016).…”

“…2), near the city of Al-Madinah Al-Munawarah (hereafter referred to as . Only a few individual eruptions from Harrat Rahat have been studied in detail, with particular emphasis on the youngest appearing scoria cones and lava flows within the volcanic field's more northerly confines close to Al-Madinah (e.g., Camp et al, 1987;Murcia et al, 2015Murcia et al, , 2017. All remaining studies have dealt with this volcanic field in its entirety and have primarily focused on magmatic processes (e.g., Camp and Roobol, 1989;Moufti et al, 2012Moufti et al, , 2013.…”

“…Basalt, hawaiite, and mugearite magmas all predominantly vented in Hawaiian and Strombolian eruptions that built scoria cones and lava flows that can reach >20 km length. Maar craters and fine-grained mafic tuff rings indicate phreato magmatic activity through interaction with groundwater (e.g., Camp and Roobol, 1989;Murcia et al, 2015Murcia et al, , 2017, although such features are restricted to the high-standing main vent axis at >10 km south of Al-Madinah. Air-fall tephra deposits are rarely preserved, which is likely the result of their small volumes, coupled with a dry, unvegetated environment where eolian and fluvial processes quickly remobilize fine-grained unconsolidated materials.…”